Electrostatic masters are first imaged by one of several known ways and then are placed on a plate cylinder of an off-set duplicating press. The overall surface of the plate is then treated with an aqueous wet-out or fountain solution which wets all portions of the plate except those areas which have been imaged or are water repellant. The press inking rolls then pass over the surface of the plate and deposit a film of ink only on the ink-receptive imaged areas. In the printing operation, the ink from the imaged areas is transferred in reverse to a rubber off-set blanket which in turn prints directly onto a paper sheet so as to form a copy.
Because of the pressure imposed on the masters by the multiple rolls with which the master comes in contact, the aqueous fountain solution tends to be forced into the master requiring a high degree of water resistance necessary to prevent water absorption. Absorption of water into the base paper is likely to cause fiber swelling and dimensional expansion in a cross-machine direction, in turn causing buckle or what is known as cockle of the master. As the master enters various nips on the printing press, the cockle is flattened, creating a crease; this crease then picks up ink which reproduces on copies, causing a streak.
Co-pending application Ser. No. 315,287, filed Oct. 27, 1981, now U.S. Pat. No. 438,946, assigned to assignees of the present invention, describes the use of alkylene/unsaturated carboxylic acid copolymers for the formation of a barrier coat for a lithographic master. A preferred such copolymer is ethylene acrylic acid, which, following application from an aqueous solution and drying, forms a continuous water resistant barrier film suitable as a barrier coat for a conventional photoconductive topcoat.
This class of compounds and most other water resistant polymeric film forming materials are very dielectric, and barrier coats for electrostatic masters require a certain conductivity. As disclosed in co-pending application Ser. No. 315,287, it is a feature of the invention thereof that the class of compounds is so water resistant that such compounds can be applied in relatively low coat weights, sufficiently low so as to not adversely affect conductivity, and still provide a functional degree of water resistivity.
However, particularly for medium and long run liquid toner masters, for instance for run lengths over 2,000 copies, it may be desirable to employ heavier coat weights to the extent that a conductive agent is required. However, most effective conductive agents are either incompatible with alkylene/unsaturated carboxylic acid copolymers, or are insufficiently conductive to provide the degree of conductivity necessary for electrostatic masters.
Prior U.S. Pat. No. 3,798,032 to Lewis S. Miller, describes the formation of electroconductive coatings for copy sheets employing monomeric quaternary ammonium compounds such as choline chloride as the conductive agent. No reference is made in the patent to the use of such conductive agents in a barrier coat for electrostatic masters. The patent does make reference to barrier additives, but these are for the purpose of solvent hold-out and to prevent migration of conductive agents into the zinc oxide topcoat. Water resistance is not a criteria in the manufacture of copy paper and, in fact, one would expect that such monomeric ionic materials as choline chloride would normally have an adverse effect on water resistance. Although the patent mentions that both hydrophilic and hydrophobic polymers may be used in the formation of the coating, it is indicated that the former (hydrophilic), which have an affinity for water and are undesirable for electrostatic masters, are preferred for the copy paper which is produced.
Even the hydrophobic polymers described in the patent are said to be emulsified polymers, which would be hydrophilic on their surface so as to be in the emulsion state prior to application to a paper surface. At the point of drying, it is known that the coalescence of particles takes place and water soluble conductive materials may form into isolated pockets. The consequence of this is to have random dielectric and conductive areas eventually leading to image mottle. As such, it is likely that the components of the film will thus not be in a random admixture without any chemical or hydrogen bonding between the conductive agents and film forming polymers.